1. Field of the Invention
The present invention relates generally to current switches and, more particularly, to switches in which the switched current varies over a wide range.
2. Description of the Related Art
Although a majority of transistor current switches are used to switch a current having a substantially fixed magnitude, an important need exists for switches that can properly switch variable currents. Conventional current switches may exhibit significant timing errors in high-speed switching applications because their signal transit time varies excessively with respect to the current magnitude.
Transit time through a switching transistor is related to its transition frequency f.sub.T which is the frequency at which the short-circuit, common-emitter current gain falls to unity. It has been shown (e.g., see Paul R. Gray and Robert G. Meyer, Analysis and Design of Analog Integrated Circuits, John Wiley and Sons, New York, 1993, pp. 30-45) that a transistor's f.sub.T is related to its device capacitances (e.g., base-emitter and base-collector junction capacitances) and its transconductance.
Because transconductance is a function of emitter current, f.sub.T approaches a constant value at high currents. At low current values, device capacitances dominate and, accordingly, f.sub.T falls with decreasing current. Other effects (e.g., increased base transit time due to high-level injection) cause f.sub.T to fall again at very high currents.
These relationships are illustrated in the graph 10 of FIG. 1 which shows a plot 12 of an exemplary transistor's f.sub.T as a function of the transistor's emitter current. Because device capacitances are directly related to device size, a switching transistor can be "sized" to optimize its performance when switching a specified current magnitude. Accordingly, the exemplary transistor has been sized to maximize its f.sub.T at an emitter current of .about.100 milliamps and it can be seen that f.sub.T has fallen to nearly 10% of its peak value at an emitter current of 1 milliamp.
The dynamic range of a variable-current switching device is typically defined as the difference between the minimum and maximum currents that can be switched without exceeding a specified signal transit-time window. In a switching application in which the signal transit-time window is narrow and the minimum current is zero, the wide variation of f.sub.T in FIG. 1 indicates that the exemplary transistor will have a limited dynamic range.